I know this isn't exactly apples to apples for comparison purposes, but Adam Timmerman was a 7th round (230 overall) pick, quite near UDFA, and he was quite a contributor to this particular organization for many years. I actually miss him.
IMO, Allen will be a decent OG at this level. He just needs some seasoning. Much like I think Trautwien is a future RT for this team. The Rams pro personel wanted him for a reason and I think he'll also develop.